The recall of pet foods contaminated with a chemical used to make plastics has grown to include both wet and dry products, even as investigators remain uncertain about why the substance would be fatal to dogs and cats, The Associated Press reported Saturday.

Hill's Pet Nutrition became the first company to recall any dry pet food, saying its Prescription Diet m/d Feline dry cat food was made using wheat gluten purchased from a U.S. supplier of the vegetable protein source. That same unnamed company also supplied the imported Chinese wheat gluten to Menu Food, which earlier this month recalled 60 million containers of the wet dog and cat food it produces for sale under nearly 100 brands.

“We were informed of the recall last night (Friday) and started pulling it off our shelves,” said Dr. Gail Counts, who operates both Shawnee Animal Clinic and Sierra's Haven Animal Shelter. “We are in the process of contacting all of our clients that purchased the Hill's dry Feline m/d Weight Control product and asking them to bring them back in because Hill's is going to refund their money.”

Counts said it was the first time she had heard of dry food being named in a recall.

“It makes you skeptical about any of the dry cat foods now,” she said. “As a consumer, I would look for the ingredient ‘wheat gluten' - I would try to avoid feeding it to my cat. In fact, they should stop feeding it, and keep any unused product.”

Counts said she has seen 10 cases of animals showing signs of illness since the recalls were first announced a week ago.

“Three have been severe,” she said. “One died, and two more are pretty severe. When this occurs, they die from renal failure.”

Counts said cat owners should not wait for symptoms.

“Even if they are not showing signs, they should definitely bring them in for blood tests,” she said.

Federal testing of recalled pet foods and the wheat gluten they contained turned up the chemical melamine, but failed to confirm the presence of aminopterin, a cancer drug also used as rat poison, according to the Food and Drug Administration.

Cornell University scientists also reportedly found melamine in the urine of sick cats, as well as in the kidney of one cat that died after eating some of the recalled food.

Earlier, New York State Food Laboratory identified aminopterin as the likely culprit in the pet food. But the FDA said it could not confirm that finding, nor have researchers at University of Medicine and Dentistry of New Jersey when they looked at tissue samples taken from dead cats.

“The important point today is that the source of the adulteration has been identified and removed from our system,” said Paul Henderson, Menu Foods chief executive officer and president. Henderson suggested his company would pursue legal action against the supplier.

“The pet food that we have manufactured after March 6 is safe and healthy,” he said. “How do we know this? First, it contains no melamine. Secondly, it contains none of the suspect wheat gluten. Thirdly, all of the testing that has been conducted, including the routine taste tests that were under way before the discovery of this problem, have demonstrated that those products not associated with the suspect wheat gluten performed very well and in a manner consistent with historic norms.”

Pet owners can get more information, including the specific brand names of products involved in the recall, by going to www.menufoods.com/recall/.

The Associated Press contributed to this story. FRANK LEWIS can be reached at (740) 353-3101, ext. 232.